R4 insulation is 1/4W per m^2 per deg C of temp difference. Adding a second layer makes it R8, 1/8W per m^2 per deg C of temp difference.

OK so say you have two sq m, one is R4 the other is double-thick R8. The total leakage is 1/4+1/8 =0.375W per deg.

Let's say that instead we split the second 1 sq m sheet from the thick half into two equal pieces and distribute them to make two R6 panels, the total leakage is 1/6+1/6=0.333W

The problem is the metal alone: Low carbon steel has a thermal conductivity of 54 (W/m K). 24ga (thick high quality door) is 5.695mm=0.005695m for an R-value of 1.0546296296296296296296296296296e-4. (I THINK I have this right).

So bare steel being an R-value of "close to zero" is worse than it sounds. Heat transfer is reciprocal, so 0 would mean "infinite". In reality the steel surface does get hot but heat transfer into the room is limited to just radiation and conduction into adjacent air (air at the door surface will get very hot, but it doesn't circulate rapidly).

So, two R4 panels on top of 24ga steel is 0.5W per deg C.

Heat transfer through 1 panel of steel beside one panel of R8:

1/1.054e-4 + 1/8 = 9482 + 0.125 = 9482.125

Like I say, in reality the leakage of heat through steel has other limits, but the basic math direction is that an unprotected sq meter is so high compared to simple R4 that only that one unprotected sq meter will matter.

The leakage through the insulated half is already almost nothing, relatively speaking, compared to the exposed panel... or many of them. So doubling the insulation value to halve the leakage across the insulated only halves the leakage on the part that was too low to matter to begin with, and there's tons of heat flowing across the uninsulated portion.

Get a better garage door opener. Insulation is expensive and keeping your garage at the right temp is of great value. You want that insulation.